I'm just re-re-reading Heirs of Empire, and I too would love to read another few books in the series. But something both prequel and sequel.
Anything to do with Aku'Ultan will have to be centuries into the future. Personally, I would like to read about the Fifth Imperium encountering either a new alien race or another human empire.
For a prequel book, I would love to read about one of those ships that just happened to 'disappear' as the bio-weapon was doing its infernal job.
I actually sent a short story to DW's email address a few years ago on the subject. (All rights and properties dealing with the Dahak series are of course belonging to David Weber. Anything written by me using that 'universe' belongs wholly and completely to David Weber.) I didn't hear anything back, but then I didn't expect to anyway...
And no matter what, at least from reading all these posts, Mr. Weber knows the impact his stories have made upon us!